Electronic packaging is the method whereby a discrete semiconductor is encased or encapsulated to protect it from environmental contamination (e.g., moisture and particulates), and is connected to the external circuit. Packages combine various materials chosen for their electrical, mechanical, or thermal properties, which are then assembled and bonded by standard techniques. The optimum packaging for a particular semiconductor device is determined by the desired performance characteristics and reliability requirements.
In a typical package a semiconductor die is attached via solder or conductive epoxy to a substrate. The die pads are connected to external leads by wire bonds. The entire circuit is then covered with a ceramic or metal lid or encapsulated in plastic or epoxy, thus forming one of several configurations.
The perfect electronic package might be described as one which is "transparent" to the circuit; that is, the package in no way alters the signal or waveform of energy either entering or leaving the semiconductor. Thus, any changes to the signal or waveform would be due solely to the action of the semiconductor.
However, the perfect package does not exist due to inductive, capacitive, and resistive properties inherent to the packaging materials themselves, and due to the physical arrangement or geometry of the materials in relation to one another in the assembled package. These deleterious phenomena are variously described as strays and parasitics. They are not critical in most low power, low frequency semiconductor applications, but at high power (1 KW) and higher frequencies (200 KHz) they pose serious obstacles to device performance.
With recent advances in the understanding of solid state physics and improved semiconductor fabrication techniques, manufacturers now offer devices inherently capable of high power, high frequency operation. These offer the promise of improved reliability, efficiency, and performance in a wide range of important high power technologies now based on antiquated tube designs, such as lasers, telecommunications, radar/sonar, radio transmitters and airborne switch mode power conversion. However, the potential contribution of the new devices to these industries has been impaired by the lag in advances in packaging technology. This fact is widely acknowledged by experts in the solid state power electronics industry.
High frequency and high power each present unique set of packaging design problems. Due to the inherent inductance of a conductor, as the current changes at high frequency the concomitant changes in magnetic flux creates a reactive term which resists the current flow. High power is the product of high voltages and high currents, thus the packaging materials are subjected to severe electrical and thermal stresses.
When high frequency and high power are combined, the problems multiply by orders of magnitude. Parasitic packaging elements, which are inconsequential at low power or low frequency, can severely degrade the performance of the semiconductor operating at high power and high frequency. Energy loss due to inductance is directly proportional to the current, so as the current rises, large B-fields emanate from the conductor which store energy. To further compound these reactive terms, as the current flows from the external circuit to the semiconductor itself, it will encounter changing impedances through various packaging materials and physical geometries.
Each segment has a characteristic impedance, and if there are abrupt changes from one segment to another, a major discontinuity results which causes an even larger reactive term. Also, as the frequency increases, if the device is at significant power, due to internal resistive and reactive terms, it will begin to dissipate heat that can destroy it if the package cannot remove the heat rapidly enough. As waveforms interact with each of these terms, they are gradually degraded.
A number of conventional packages exist for high power, low frequency semiconductors and for low power, high frequency semiconductors.
For low frequency, high power applications, two basic types of packaging have dominated since the 1960s: the TO-3, widely used in military applications, and the TO-220 style packages. The TO-3 consists of a cylindrical metal case on top of a diamond-shaped metallic substrate of steel, nickel plated copper, or a kovar alloy, with two rigid round leads emerging from the bottom. The TO-3 style device is held by two screws to a heat sink. The TO-220 style package is a flat, rectangular shape with a metal tab lead for heat dissipation on one end and three copper leads for electrical connections on the other. It is attached by a single screw to a heat sink. Most other prior art packaging configurations are variations on these two.
Neither the TO-3 nor the TO-220 style packages are suitable for high frequency operation. The round, rigid kovar leads and single, heavy gauge bond wire attachments exhibit severe inductive terms that prohibit fast switching. Often an additional inductive term is added as the leads pass through the metal base. Furthermore, their numerous thermal boundaries prevent rapid efficient heat dissipation. Because the coefficient of expansion of these metals is poorly matched to silicon, it is not uncommon for the metal to expand sufficiently to crack the semiconductor.
The primary packaging technique utilized for low power, high frequency semiconductor devices is the use of an RF or microwave "stripline" design characterized by one or more flat leads. Flat leads are used to mate with the broad stripline terminating on the circuit board and for another important reason. The inductance of a conductive path is proportional to its enclosed volume. By flattening the lead and placing it near the return path or ground plane, the inductance is reduced. Also, at high frequencies, current tends to migrate to the surface of the conductor, a phenomenon known as the skin effect; thus a broader lead can carry more current at higher frequencies. The leads are usually copper or a copper-kovar alloy. These devices are typically much smaller than high power packages. The low power semiconductors are very small so only moderate heat-sinking is required.
The problems noted above are observed in many types of semiconductor packages. They are particularly pronounced in regards to pulsed power applications.
Pulsed power is a method of storing energy over a long period of time and releasing that energy in a very short period of time at a very high power level. An increasing number of tasks rapidly emerging in the scientific community are demonstrating the need to generate bursts of power at speeds approaching the limits of current pulsed-power technology.
The generation of energetic pulses has previously depended strongly on non-solid-state switching technology when speed and voltage requirements are less than a few nanoseconds and greater than a few hundred volts, respectively. Switching requirements of greater than 10 kW in less than 3 nS, for example, limit the circuit designer to only a few technologies, none of them particularly simple or inexpensive. The addition of high repetition-rate requirements compounds the problem, and will generally limit the designer to a vacuum tube approach if the repetition rate and/or rise time are outside available thyratron capabilities.
The emergence of power MOS field-effective transistors (FET) over the past few years has created a revolution in the design of mass-produced circuits such as switching power supplies. Circuit designers have been able to use them to achieve switching speeds of 20 to 50 nS in power supply applications, which is a significant improvement over conventional bipolar transistors.
The key element in a pulse generator circuit is the switch which provides the fast rise-time output. Several technologies have been available for fast, high-voltage applications in the past. However, for work under 5 nS, the choices are somewhat limited. Gas switches (spark gaps) and krytrons, for example, have been popular for their high performance and low cost. Their use is often precluded, however, by high jitter and poor operational lifetime. More recently, the bulk-semiconductor photoconductive (Auston) switch has emerged as an effective high-voltage switch. This device relies on the photon-induced generation of carriers within a bulk-semiconductor material. Typically, the Auston switch requires a complex pulsed laser system for turn-on. Some other approaches rely on rise-time improvement using various magnetic techniques, but waveform is difficult to control with these methods and an active pulse generator must be provided. Avalanche-mode transistors continue to be used and indeed present an attractive solution to many engineering problems. They are, however, power limited and often exhibit reliability problems. Such difficulties are compounded when multiple devices are used to attempt to circumvent the power issue. For completeness, the hydrogen thyratron is mentioned, but it is limited to a rise time of around 3 nS. Most thyratrons are no faster than 15 nS.
This often leaves the vacuum tube as the only viable switching element available for rise-time requirements under 3 nS and greater than 10 kW and for circuits with repetition-rate conditions requiring low jitter. Such specifications would describe a device which could function as a Pockels cell driver, for instance. Pockels cells are high speed electro-optic devices used as shutters in laser systems. For such a speed requirement, component selection is generally limited to microwave planar triodes such as the Varian/Eimac Y-690. High speed circuits used with these devices tend to be complex because the circuit-induced multiplication of the input capacitance of the triode (known as the Miller Effect) limits power gain. The vacuum-tube current for pulse applications also is limited to around 12A/cm.sup.2 of cathode area. Higher currents are possible but tube lifetime may be degraded.
In the past few years, power MOS has solved many power circuit problems and created a new generation of high-frequency switching power supplies which are small and lightweight. This improvement in power-supply performance still does not satisfy the specialized needs of high voltage pulse generators requiring output voltages of up to 10 kV and with rise-time requirements sometimes below 1 nS. Such pulsers find use in lasers and other optical systems, radio frequency modulators, other sensor drivers, and a variety of miscellaneous tasks continually emerging in the scientific community.
All power MOS is implemented as an array of thousands of transistor cells on each MOSFET chip; these cells are approximately 1 to 2 square mils in area. The switching speed inherent in any given cell is extremely fast, typically on the order of 200 pS. However, overall device switching speed of packaged chips is significantly slower.
Hybrid high power electronic components are required for a wide range of circuit designs, especially in power conversion, high frequency RF, and microwave applications. A hybrid power circuit consists of a semiconductor die attached via solder or conductive epoxy to a substrate. The die pads are connected to external leads by wire bonds. The entire microcircuit is then encapsulated, thus forming one of several packaging and interconnect configurations which are available to interface the hybrid circuit to the external circuit. Typically, these packages are either plastic or metal.
There has not heretofore been provided packaging techniques which enable metal oxide semiconductor devices to be used in very fast switching applications high power pulse generators.